Ink jet printers are becoming an increasing popular type of device for recording permanent images on paper. Ink jet printers operate by directing a stream of minute ink droplets at the paper so as to produce a distinct pattern of individual ink dots. By selectively forming ink dots on paper, and by regulating the number of dots formed on the paper, an ink jet printer can be used to create almost any type of print: text; graphics; or images. This capability has made it attractive to attach ink jet printers to computer systems that produce both textual material and images simultaneously. This is because a properly programmed ink jet printer can be used to produce a complicated image and a detailed description of the image on the same page.
Moreover, many ink jet printers are capable of discharging multiple colors of ink so as to generate quality color figures and images. This capability has contributed to their popularity since computer systems that can generate multi-color video output in the form of graphics and images are becoming increasingly common. These computer systems require printing devices that can produce permanent images of the output they generate. The ability of ink jet printers' to produce text and images in color has also made them useful for desk top publishing which allows a small user to efficiently, economically and rapidly produce publications that contain textual material that is accompanied by color images.
Like many other liquid discharging systems, ink jet printers must be "primed" before they can operate. Typically, this is accomplished by applying pressure to a reservoir where the ink is stored prior to its discharge through the jets. The pressure forces ink through the jets up to the jet openings so as to fill the jets with ink to insure proper performance, including the consistent discharge of ink droplets of substantially identical size whenever the ink jets are activated.
The proper operation of the jets, even after priming, is often adversely affected by residual matter that is frequently found inside the jet passageways. This residual matter includes deposits of dried ink that remain in the jets, such as at the end of the day after the printer is turned off. In addition to possibly blocking the orifice of the jets, these deposits can cause bubble-forming turbulence to develop during the priming process. The bubbles attach themselves to the walls of the jet passageways, and are often not discharged therefrom by subsequent purging (i.e., clearing the ink passageways of air or debris by forced flow of ink) or repriming. As a result the jet passageways will not be completely full of ink, and consequently ink may not jet from the jets, or it may be jetted at varying velocities therefrom, or ink droplets of substantially different sizes may be discharged.
A variety of inks are used in ink jet printers, including inks that are normally liquid at room temperatures and above (hereinafter referred to simply as "liquid" inks) and those that are normally solid at room temperatures but that are heated to elevated temperatures to liquefy them for jetting (the so-called "hot-melt" or "phase-change" inks). Hot-melt inking systems are used, in part, because the ink they discharge solidifies rapidly on contact with the paper and the forms ink dots with very sharp optical edges so the resulting images are of very high quality. Hot melt inks also have exceptional true color mixing properties which is an important characteristic for color printers that typically have three base color inks, plus black, that are blended together to print a very large spectrum of intermediate colors.
Hot-melt ink jet printers face similar priming and purging requirements as do printers that use conventional liquid inks. In addition, the solidification of the ink, that accompanies shut-down, or deactivation, of the printer at the end of the day generates air bubbles from contraction or solidification and subsequent remelting on startup. This can fill the jet passageways with a froth head of ink heavily embedded with air bubbles.
A froth head is very difficult to remove from the jet passageway. Attempts to clear a froth head from an jet passageway are frequently unsuccessful because the froth head comprises bubbles which attach themselves to the walls of the jet passageways and are difficult to remove therefrom. The reason the bubbles cannot be discharged by priming is that a jet opening acts as a restricter to prevent adequate removal of bubbles from the larger diameter jet passageway. Attempts to discharge the froth heads from passageways by forcing a stronger purging flow of ink through them are similarly unsuccessful for the same reason. Moreover, when a strong purging flow is performed, relatively large amounts of ink are discharged from the jet; this reduces the supply of available ink, and the large volume of ink discharged unnecessarily wets the jet faces with ink.
Solid ink deposits that form inside the jet passageways may also cause bubble-forming turbulence to develop when the jets are primed. These deposits can form because ink becomes trapped in small indentations in the passageways that are invariably formed when the jets are manufactured. The ink that dries in these indentations forms solid deposits. This problem is especially a problem with printers having hot melt inking systems or other printers having a heated ink delivery system. The ink in these printers is heated, and thus can be "cooked" into the indentations as the volatile components of the ink are evaporated off. The turbulence induced by these solid deposits of ink in the jet passageways causes bubbles to form during the priming process and remain thereafter, leaving jets with less than full heads of ink.
The effectiveness of priming may also be reduced if the interior walls of the jets are dry. This is because the dry walls slow the priming flow near the walls in comparison to the flow in the center of the jets and causes erratic meniscus behavior at the head of the priming flow. As a result turbulence, and subsequent bubbles, develop adjacent the jet walls during priming, and the bubbles remain afterwards, thus reducing the effect of the priming operation.
Moreover, most ink jet printers commonly have problems caused by dirt accumulation. The dirt is from paper dust, small ink droplets that are redeposited on the faces of the jets, and other bits of matter that adhere to the heads of the ink jets adjacent their openings. These bits of matter develop into large agglomerations that project over the jet openings. Sometimes, if the agglomerations are unusually large, they may even extend into the jet openings. These agglomerations of dirt cause a problem because they deflect the stream of ink droplets that are discharged from the jets. In some instances the agglomerations on the face of a jet may be so large that the discharge of ink therefrom is completely inhibited. As a result, the ink is inaccurately deposited on the paper and the quality of the resulting image produced is degraded. Furthermore, if the agglomerations of dirt become too large, some of the dirt may rub onto the paper and smear it.
Current apparatuses designed to minimize the problems associated with ink drying and bubbles forming within the jets of ink jet printers typically include a capping mechanism that operates in conjunction with a purging system. Examples of these systems are disclosed in U.S. Pat. Nos. 4,144,537 and 4,177,471. These patents disclose the use of capping apparatus that include some sort of cap that is urged against the ink jet opening, or which the ink jet openings are urged against. The capping apparatus is used to keep the jets covered while the printer is not in use. A purging mechanism is also provided to flush and fill the jets prior to activating the printers. A limitation of these devices is that all they do is provide a capping mechanism that is supposed to maintain the meniscus inside each of the ink jets. If the seal the cap forms around the jet breaks, the ink meniscus may slowly break, leaving the walls of the ink jets with a coating of ink that will inhibit the purging process. These devices also do nothing to alleviate the problems caused by ink deposits forming on the jet walls, or to otherwise prepare the ink jets for priming. Furthermore, only one of these apparatuses, the one disclosed in U.S. Pat. No. 4,144,537, includes a means to clean the heads of the ink jets; and the cleaning means disclosed is only an indirect system where the surface used to cap the jets is cleaned after the process. This cleaning mechanism does not insure that entire surfaces of the ink jet faces are substantially cleaned.